This invention relates to a sealing gasket adapted to be interposed between the cover and the frame of a roadway manhole accessing an underground system, particularly a municipal sewage disposal system.
Local ordinances frequently require that roadway manholes accessing sewage disposal systems be sealed to prevent rain water or the like from trickling into the system and thus overloading sewage treatment stations. One approach has been to interpose a simple, sufficiently flexible gasket, for example made of an elastomer having a hardness of between 45.degree. Shore and 60.degree. Shore, between the cover and the frame of the manhole so that it fits snugly against their rough cast surfaces. If the cover is not locked to the frame, however, for example by appropriate edge clamps or a bayonet coupling, the movements of the cover within the frame tend to shear the gasket and thus disrupt its sealing effect. This problem may be offset or at least partially overcome by making the gasket from a harder elastomer on the order of 70.degree. Shore to 80.degree. Shore, but such increased hardness detracts from its sealing effect.
Another approach is to lock the cover to the frame to prevent it from moving, thus reducing the shearing effect on the gasket. For example, U.S. Pat. No. 4,203,686 teaches a roadway manhole in which the cover is held in the frame by a locking ring laid against the upper surface of the cover and having bearing surfaces located opposite corresponding surfaces of the frame. The gasket is interposed between the frame and the cover, and is compressed when the ring is rotated to lock the cover to the frame. Such a construction requires a costly and difficult to fabricate locking mechanism, however, and moreover the frictional forces between the various components are often insufficient to prevent the cover from rotating and thus abrading the gasket upon the passage of a vehicle.